Arc plasma generator starting method utilizing a displaceable valve communicating with atmosphere



3,356,892 ARC PLASMA GENERATOR STARTING METHOD UTILIZING A DISPLACEABLE ML nu L H E U WI em. V I I I & I w V 1 i m Dec. 5, 1967 EA. BUNT ETAL VALVE COMMUNICATING WITH ATMOSPHERE Original Filed Jan. 10, 1963 mum wvw

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EDGAR A. BUNT LLOYD o. KAUFFMAN, Jr.

HERMAN L. OLSEN SPENCER o. RAEZER INVENTORS wumDOm m 0 P317: QmNEDMmMEA United States Patent 4 Claims. (Cl. 315-111) ABSTRACT OF THE DISCLOSURE Generally speaking, the present invention relates to an improved method for starting arc plasma generators or the like, at relatively high chamber pressures and in the presence of gases entering the chamber at relatively high rates of mass flow. More particularly, the present invention envisions the use of a variable orifice valve communicating the inside of the are chamber with a large ballast volume, such as the atmosphere. In accordance with the proposed starting method, gas flow at the desired rate of mass flow is initiated into the arc chamber and the valve is opened to place the chamber in communication with the ballast volume. The are is then struck between the electrodes and since the arc chamber is then connected to the ballast volume, the instantaneous pressure rise within the chamber is controlled to such an extent that there are no excessive surge voltage demands on the electrode energizing circuits; which surge voltage demands, if permitted to exist, could cause premature extinguishing of the arc. After the arc is struck, the valve is then closed and the input gas flow rate increased if desired.

This invention relates generally to are plasma generators. More specifically, it relates to an improved, continuously operated arc plasma generator incorporating novel apparatus to facilitate starting thereof, and to a method for starting said generator at relatively high chamber pressures and in the presence of gases entering the chamber at relatively high rates of mass flow.

This application is a division of copending US. application Ser. No. 250,722, filed Jan. 10, 1963 by the present applicants.

The are plasma generator of the invention is designed to provide a plasma discharge, which for purposes of this invention is defined as a partially ionized, gaseous-like mass derived by passing air or a similar substance through a high energy electric arc. The plasma discharge from the generator is therefore a collection of neutral particles, ionized particles, and free electrons, all of which are free to move and have mutual collisions. The temperature of the discharge produced by the generator is of the same order as the indicated temperature at the surface of the sun.

Numerous applications have been found for the plasma discharge of an arc plasma generator, including the testing, cutting and welding of materials, metal spraying, chemical processing, and as a heat source for equipment such as windtunnel facilities capable of simulating hypersonic flight. Because the subject plasma generator is capable of operating well above the maximum capabilities of existing chemical and storage heaters, such as heaters of the pebble bed type, it presents new capabilities for both industrial and scientific purposes.

Several general types of arc plasma generators have been devised, the type to which this invention relates typ- "ice ically comprising a pressure vessel containing a chamber within which is mounted at least a pair of substantially identical, parallel, closely spaced ringlike electrodes, said electrodes being insulated from each other and from the vessel. A high voltage is applied to the electrodes, and an arc is struck therebetween. A suitable pressurized gas is admitted to the chamber, is passed through the arc, whereby it is transformed into plasma, and is then discharged from the vessel through a suitable orifice.

In certain applications of arc plasma generators it is desirable to strike the arc, and thus commence operation of the generator, While the arc chamber is pressurized and while gas is flowing into said chamber. However, in the past it has been found in arc plasma generators of the type to which this invention relates that under these conditions the arc would frequently blow out almost immediately after it was struck.

It has now been discovered that the cause for this blowing out, or extinguishing, of the arc is a sharp pressure rise occurring at the instant the arc is struck, which pressure rise is of such a short duration and of such a large magnitude that it is normally unmeasurable. The instantaneous pressure rise requires an instantaneous increase of like magnitude in the voltage sustaining the arc, which voltage demand can normally not be met; thus, the arc is extinguished.

In the plasma generator of the present invention the extinguishing of the are has been controlled by mounting an adjustable orifice valve in the pressure vessel. According to the novel are starting method of the invention, gas flow at the desired rate of mass flow is initiated into the are chamber after said valve has been opened to place the chamber in communication with a large ballast volume, such as the atmosphere. The are is then struck in the usual manner, the critical period for pressure rise occurs, and after a brief interval (one second or a little longer) the valve is closed; by utilizing the large ballast volume during the striking of the arc the instantaneous pressure rise is controlled to such an extent that the surging voltage demand can easily be met. Thus, the are plasma generator of the invention may be readily started without danger of the are being extinguished.

It is noted that the utility of the plug valve method of starting is particularly good when a relatively large rate of gas mass flow (of the order of one lb. per second) is required to flow through the arc, as a high mass flow augments the required voltage. If sufliciently high voltage is not available, successful starting can also be elfected by firing on a temporarily reduced mass flow; this, however, unnecessarily increases the enthalpy and temperature of the gas beyond operating requirements and may even lead to breakdown of the cooling walls. All of this is avoided in the present invention by keeping the mass flow constant and using the plug valve to discharge the pressure pulse occurring upon firing of the are.

It is an object of this invention to provide an improved method and apparatus for starting an arc plasma generator.

Another object of this invention is to provide a method for starting an arc plasma generator in the presence of relatively high chamber pressures and gases flowing into said chamber at relatively high rates of mass flow.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a side elevation of an arc plasma generator incorporating the improved starting method and apparatus of the present invention, showing the variable orifice starting valve in its fully opened position and also showing the flow paths of the input gas and the output plasma;

FIG. 2 is an enlarged longitudinal section through the variable orifice starting valve, the valve being shown in its fully closed position; and

FIG. 3 is a simplified diagrammatic representation of spaced apart electrodes suitable for use in the illustrated embodiment of the present invention, with an arc struck therebetween.

Referring now to FIG. 1 of the drawings, one form of an arc plasma generator is illustrated, incorporating the proposed starting method and apparatus of the present invention. The detailed construction of the illustrated arc plasma generator is more completely described in our co pending parent application, Ser. No. 250,722, filed Ian. 10, 1963, and generally comprises a cylindrical pressure vessel 1 which is formed by a flanged, hollow cylindrical vessel 2 having closure plate 24 and 26 attached to the opposite flanged ends 4 and 6 thereof, by bolts 22, washers 38 and 40, and nuts 36.

A pair of electrodes, preferably of the configuration disclosed in our co-pending parent application, Ser. No. 250,722, filed J an. 10, 1963, is supported in confronting, spaced-apart relationship within the interior arc chamber of the generator, as is described in our above-mentioned parent application, by electrode supports 126 and 152 and clamps 134 and 154 of FIG. 1. This interior arc chamber of the generator is also connected, by means of circumferentially spaced bores, to a suitable source of gas and is moreover provided with suitable water jacketing adapted to be connected to a source of cooling water, via bores such as that designated at 17 in FIG. 1. The right-hand end of the arc generator illustrated in FIG. 1 is adapted, in accordance with the teachings of our copending parent application, Ser. No. 250,722, to be secured to the entrance duct of a hypersonic wind tunnel, for example; whereas, the left-hand end of the arc generator is adapted to communicate the interior of the generator with the atmosphere, via the variable orifice valve assembly generally indicated at 206 in FIG. 1.

While arc plasma generators similar to that disclosed in our co-pending application Ser. No. 250,722, filed I an. 10, 1963, have been found to operate satisfactorily once they have been started, in the past certain difficulties have been encountered in the starting process. Specifically, it has been found that in the presence of relatively high chambr pressures and mass rates of gas fiow the arc would be struck, and would then very often become extinguished. It has now been discovered that the cause for the extinguishing of the arc is the existence of a substantially unrecordable sharp pressure pulse on firing, which pulse creates an accompanying sharp voltage demand that normally cannot be met by the source. The present invention provides apparatus and a method of operation for overcoming this starting difiiculty.

Referring now to FIGS. 1 and 2, a variable orifice valve assembly is indicated at 206, and includes a cylindrical housing 208 having a flange 210 on its aft end and a cylindrical collar 212 on the forward end thereof. The collar 212 is of a size to be snugly received within a centrallydisposed socket in the aft closure plate 26.

The housing 208 has an annular groove 220 in the forward end face thereof, which is closed by a welded-inposition annular plate 222 received within a second, shallower annular groove in said end face. A plurality of axially directed, circumferentially-spaced bores 224 extend through said housing, and are in communication at their aft ends with conduits 226. One half of the conduits 226 normally function as fiuid inlets and the other half as fluid outlets, and thus cooling fluid may be made to flow through said housing.

The aft flange 210 has a peripheral, stepped groove 230 therein, which is closed by a welded-in-position, stepped annular band 232. A plurality of circumferentially-spaced conduits 234 are secured within bores in the band 232, and function to pass cooling water through the annular channel 236 within said flange. The conduits 226 and 234 are omitted from FIG. 1 for purposes of clarity.

Welded to the flange 210 and extending rearwardly therefrom are three equally-spaced, parallel support rods 238, each terminating in a threaded portion at its aft end. The free ends of the rods 238 extend through bores in a mounting plate 240, said plate being clamped in position by nuts 242 threaded on said rods and disposed on both sides thereof. A supporting plate 244 is secured to the mounting plate 240 by bolts 246, and extends downwardly to a suitable base for furnishing vertical support to the valve assembly 206.

A three-arm spider 248 is supported on the rods 238, each of said arms having a bore therethrough at its radially outer end through which one of said rods passes. The spider is secured in position by nuts 250 threaded on the support rods, and includes a central hub 252 having a bore therethrough for slidably receiving a valve stem 254. The valve stem 254 is connected at its aft end to the actuator shaft 256 of a double-acting hydraulic cylinder assembly 258, said assembly 258 being mounted on the plate 240 by stud bolts 260 and said shaft 256 extending through a bore (not shown) in said plate.

The valve stem 254 terminates at its forward end in a relatively large cylindrical head 262, said head having a recess 264 therein and a peripheral groove 266 in its exterior surface at the forward end thereof. A cup-shaped, hollow valve cap 268 is received on the forward end of the head 262 and is secured thereto, as by welding. The cap 268 includes a cylindrical portion 270 and a frustoconical nose portion 272 terminating in a rounded tip. The taper on the nose portion 272 corresponds to the taper of a frusto-conical valve seat 274 formed in the aft end face of the housing 208 about the bore 228.

The valve head 262 has a pair of angled bores 276 extending through the rear end thereof, within which are secured a pair of angled tubes 278; the tubes 278 lie immediately adjacent and extend parallel to each other within the valve head, and project forwardly nearly into contact with the inner surface of the frusto-conical cap portion 272. The valve head 262 also has a pair of diametrically opposed, radially-directed bores near the aft end thereof, within which are secured conduits 280 (not shown in FIG. 1). Cooling water is injected into the valve head through the tubes 278, is discharged against the frusto-conical nose surface on the cap 268, and then flows out of said head through the conduits 280; thus, the valve head is cooled and is protected against being melted by the high temperature gases existing within the are chamber, particularly when these gases are flowing out through the orifice before the cap has shut tightly.

The valve head 262 may be moved axially by the double-acting hydraulic cylinder 258 from its fully open position, shown in FIG. 1, to the fully closed position shown in FIG. 2. As the frusto-conical cap 272 moves toward and away from the frusto-conical seat 274 a variable area orifice is provided, which orifice places the interior of the pressure vessel 2 in communication with the atmosphere through bore 228. By properly operating the double-acting hydraulic cylinder 258 immediately after the arc is struck, premature extinguishing, or blowout, of the arc during starting may be eliminated.

In operation, the hydraulic cylinder 258 is first activated to move the valve head 262 away from the valve seat 274 until the area of the variable orifice defined there between is at a maximum, thus placing the arc chamber in communication with the large ballast volume which is the atmosphere. Gas from a suitable source is then admitted into the arc chamber, via input lines such as those illustrated at 176a and 178a, at a relatively high rate of mass flow, say one lb. per second by means of a suitable control valve or the like. The are is then struck between the electrodes, and immediately thereafter the hydraulic cylinder is actuated to close the variable area starting orifice; the valve head is moved at a uniform rate such that the orifice is completely closed after a period of a few seconds, two seconds being a typical closure period. If required, the mass flow rate of the gas may then be increased to any other desired operating value, thereby establishing the desired operating pressure Within the arc chamber. It should be pointed out here that the illustrated gas input lines 176a and 178a are typical of the gas input lines that would be connected into the circumferentially-spaced bores 176 and 178 respectively, shown in FIG. 3 of our co-pending parent application, Ser. No. 250,722, filed Jan. 10, 1963 and now US. Patent N0. 3,274,424, issued Sept. 20, 1966.

The are plasma generator starting method just described eliminates premature extinguishing of the are immediately after it is struck by opening the arc chamber to the large ballast volume of the atmosphere during the period when the arc is struck and is becoming established; the pressure surge at striking is thus damped, and the accompanying surge in voltage requirements is kept down to an easily satisfied level. The maintenance of gas flow into the arc chamber during the starting process insures proper operation of the generator, and makes possible the rapid achieving of operational arc chamber pressures.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It should therefore be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A method for starting an arc plasma generator, comprising the successive steps of placing the interior of said generator in communication with the atmosphere,

initiating a flow of gas into said generator at a mass flow rate equal to the normal operating value thereof,

striking an are between the electrodes of said generator,

and

closing off the interior of said generator from the atmosphere after said arc has been struck.

2. A method for starting a plasma arc generator as recited in claim 16, wherein said last mentioned step further includes closing off the interior of said generator from the atmosphere at a uniform rate over the period of a few second after said are has been struck.

3. A method for starting a plasma arc generator as recited in claim 2, including the additional step after closing off said generator of increasing the rate of mass flow of gas into said generator.

4. The method recited in claim 1 wherein the steps of placing the interior of said generator in communication with the atmosphere and of closing off the interior of said generator from the atmosphere are accomplished by a hydraulically operated variable orifice valve means.

References Cited UNITED STATES PATENTS 3,004,189 10/ 1961 Giannini 203 3,007,030 10/1961 Ducati 313-231.5

S. D. SCHLOSSER, Primary Examiner. 

1. A METHOD FOR STARTING AN ARC PLASMA GENERATOR, COMPRISING THE SUCCESSIVE STEPS OF PLACING THE INTERIOR OF SAID GENERATOR IN COMMUNICATION WITH THE ATMOSPHERE, INITIATING A FLOW OF GAS INTO SAID GENERATOR AT A MASS FLOW RATE EQUAL TO THE NORMAL OPERATING VALUE THEREOF, STRIKING AN ARC BETWEEN THE ELECTRODES OF SAID GENERATOR, AND 